This invention relates to gas-permeable membranes which are useful in particular for the packaging of biological materials, especially fresh produce.
Fruit and vegetables, and other respiring biological materials, consume oxygen (O2) and produce carbon dioxide (CO2) at rates which depend upon temperature and upon the particular material and the stage of its development. Their storage stability depends on the relative and absolute concentrations of O2 and CO2 in the atmosphere surrounding them, and on temperature. Ideally, a respiring material should be stored in a container having a total permeability to O2 and a total permeability to CO2 which are correlated with (i) the atmosphere outside the package (usually air), (ii) the rates at which the material consumes O2 and produces CO2, and (iii) the temperature, to produce an atmosphere within the container (the xe2x80x9cpackaging atmospherexe2x80x9d) having the desired O2 and CO2 concentrations for preservation of the material. The total permeability to water vapor may also be significant. This is the principle behind the technology of controlled atmosphere packaging (CAP) and modified atmosphere packaging (MAP), as discussed, for example, in U.S. Pat. No. 4,734,324 (Hill), U.S. Pat. No. 4,830,863 (Jones), U.S. Pat. No. 4,842,875 (Anderson), U.S. Pat. No. 4,879,078 (Antoon), U.S. Pat. No. 4,910,032 (Antoon), 4,923,703 (Antoon), U.S. Pat. No. 5,045,331 (Antoon), U.S. Pat. No. 5,160,768 (Antoon) and U.S. Pat. No. 5,254,354 (Stewart), copending, commonly assigned U.S. patent application Ser. No. 08/759,602 filed Dec. 5, 1996 now U.S. Pat. No. 6,376,030, published as International Publication No. WO 96/38495 (Application No. PCT/US96/07939), copending, commonly assigned U.S. patent application Ser. No. 08/926,928, now U.S. Pat. No. 6,013,293 and European Patent Applications Nos. 0,351,115 and 0,351,116 (Courtaulds). The disclosure of each of these documents is incorporated herein by reference.
The O2 transmission rate (referred to herein as OTR) and CO2 transmission rate (referred to herein as COTR), of a body composed of a particular material, are the amounts of O2 and CO2, respectively, which will pass through a defined area of that body under defined conditions. The total permeabilities of a container to O2 and CO2 depend, therefore, upon the areas, OTRs and COTRs of the various parts of the container.
The preferred packaging atmosphere depends on the stored material. For many materials, the preferred concentration of O2 is less than the preferred concentration of CO2. For example, broccoli is generally best stored in an atmosphere containing 1-2% O2 and 5-10% C2; berries are generally best stored in an atmosphere containing 5-10% O2 and 10-20% CO2; and cherries are generally best stored in an atmosphere containing 5-8% O2 and 10-20% CO2. In order to produce a packaging atmosphere having a high ratio of CO2 to O2, the container should have a low ratio of total CO2 permeability to total O2 permeability. The term R ratio is used herein to denote the ratio of COTR to OTR for a particular material or the ratio of total CO2 permeability to total O2 permeability of a container or part of a container.
Respiring biological materials are normally stored at temperatures substantially below normal room temperature, but are often exposed to higher temperatures before being used. At such higher temperatures, the respiration rate increases, and in order to maintain the desired packaging atmosphere, the permeability of the container preferably increases sharply between storage temperatures and room temperature.
Respiring biological materials are generally stored in sealed polymeric containers. Conventional polymeric films, when used on their own, do not provide satisfactory packaging atmospheres because their OTR and COTR values are very low and their R ratios are high. Microporous polymeric films, when used on their own, are also unsatisfactory, but for different reasons; namely because their OTR and COTR values are very high and their R ratios close to 1.0. It has been proposed, therefore, to make use of containers which comprise
(i) one or more barrier sections which are relatively large in area and are composed of materials having relatively low OTR and COTR values (e.g. are composed of a conventional polymeric film), and
(ii) one or more atmosphere-control members which are relatively small in area and are composed of a microporous film, and which provide at least a large proportion of the desired permeability for the whole container.
However, for containers of conventional size, the preferred total O2 permeability, although larger than can be provided by the barrier sections alone, is still so small that control members made of a microporous film need to be very small in area. Such very small control members are difficult to incorporate into containers, and can easily become blocked in use. In addition, the OTR of microporous films does not change much with temperature.
As described in copending, commonly assigned application Ser. No. 08/759,602 and corresponding International Publication No. WO 96/38495 (referenced above), much improved results can be obtained through the use of atmosphere-control members composed of a membrane prepared by coating a thin layer of a polymer onto a microporous film. The OTR of these membranes is such that the atmosphere-control members are of practical size. Furthermore, through appropriate choice of the coating polymer, in particular the use of a low-melting side chain crystalline (SCC) polymer, the membranes can have OTRs which increase sharply with temperature. However, although the membranes are very satisfactory for many purposes, they often have R ratios which are higher than is optimal when the desired packaging atmosphere contains a relatively large proportion of CO2. As described in copending, commonly assigned application Ser. No. 08/926,928 now U.S. Pat. No. 6,013,293, if a gas-permeable membrane is covered, on the side exposed to the air, by a relatively gas-impermeable cover member having one or more small apertures therein, the R ratio of the combination can be substantially less than the R ratio of the membrane itself.
In the continuing development of coated membranes of the kind described in the earlier applications, it has been found that their permeabilities are liable to be non-uniform and/or to change during use, particularly when using SCC polymers which give rise to high P10 values. This is apparently due to two factors. First, the SCC polymers tend to be somewhat tacky, and, therefore, to be partially removed from the microporous base film during handling. Second, their inherent permeability to gases is such that, in order to produce a coated membrane of desired size and O2 permeability, the coating must be rather thin (e.g. 2-3 microns thick). Such thin coatings are difficult to apply uniformly and are liable to be damaged during use.
This invention provides improved gas-permeable membranes which comprise
(a) a gas-permeable substrate, particularly microporous polymeric film, and
(b) a polymeric coating on the microporous film, the polymeric coating comprising a block copolymer which has a heat of fusion xcex94H of at least 5 J/g, and which comprises
(i) polysiloxane polymeric blocks, and
(ii) crystalline polymeric blocks having a melting point, Tp, of xe2x88x925xc2x0 to 40xc2x0 C.
Gas permeable membranes are particularly useful as atmosphere control members in packages for respiring biological materials, and will be chiefly described by reference to such use. It is to be understood, however, that the invention includes gas-permeable membranes which are useful for other purposes.
Even a small proportion of polysiloxane blocks reduces the tack of the polymer coating; and as the proportion of polysiloxane blocks increases, the inherent permeability of the copolymer increases, making it possible to increase the thickness of the coating so that the coating is durable and easy to apply, without an increase in the size of the atmosphere control members. For example, this invention makes it possible to prepare gas-permeable membranes which are resistant to damage through abrasion, have uniform properties, and which combine (a) a P10 ratio over at least 10xc2x0 C. range between xe2x88x925 and 15xc2x0 C. of at least 1.8, e.g. 2.0 to 2.8, particularly a P10 ratio between 0 and 10xc2x0 C. of at least 2.0, and (b) an OTR at all temperatures between 20 and 25xc2x0 C. of at least 2,325,000 ml/m2xc2x7atmxc2x724 hrs (150,000 cc/100 inch2xc2x7atmxc2x724 hrs), e.g. 2,480,000 to 3,410,000 ml/m2xc2x7atmxc2x724 hrs (160,000 to 220,000 cc/inch2xc2x7atmxc2x724 hrs).